Automatic horizontal and vertical scanning radar with terrain display

ABSTRACT

A weather radar and terrain map display system for aircraft with the terrain elevation and weather information displayed in an easy to read and quickly comprehendible presentation. The system includes an antenna for transmitting and receiving radar signals, a digitizer for digitizing the reflected radar signals, a means for storing the signals and calculating the latitude and longitude coordinates of the features from which the reflected radar signals were reflected, and for storing terrain elevation data. Flight modes are programmable to rapidly change a size of an alert region appropriate for the current flight mode. A display simultaneously shows a plan view image and vertical views of contoured terrain elevation data and the weather conditions found by the radar. The terrain and weather displays 15 are superimposed over one another to enable quick and efficient location of critical terrain and weather conditions. The system can also calculate the latitude and longitude coordinates of the radar echoes without antenna stabilization.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/613,017, filed Mar. 11, 1996 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a radar system. More specifically, the radarsystem is primarily for aircraft which use automatic horizontal andvertical scanning. The radar system is capable of displaying radarsignals superimposed with terrain elevation data in a plan view and insupplemental front and side vertical views.

2. State of the Art

Each year aircraft crash in unacceptable numbers. Investigations intothese crashes show that often the cause of the crash is not due tomechanical problems. These types of crashes are described as controlledflights into terrain. Controlled flights into terrain often result froma pilot's lack of three dimensional situational awareness. A graphicdisplay of the aircraft position relative to terrain would improve suchawareness. Pilots need information about their aircraft's positionrelative to terrain elevation in an easy to read and comprehendpresentation. The availability of detailed, world-wide data basesproviding terrain elevations make such a display possible.

Most large aircraft have Ground Proximity Warning Systems (GPWS) but theGPWS only provides a warning alarm rather than a visual display. Thispresent invention provides a simple, accurate way of displaying to thepilot a visual image of the aircraft position relative to terrainelevation as well as to weather conditions in a form that may be seen ata glance with little pilot input. A glance at the display would help thepilot distinguish a GPWS's true warning signal from a false signal.

Conventional terrain alert warning systems, such as GPWS, presume eitheran aircraft descending into terrain or flying into rising terrain andprovide "pull-up" alerts. However, some accidents occur in steepmountains where escape by pull-up is not a realistic possibility becausethe terrain rises higher or faster (more abruptly) than the aircraft canclimb for the amount of warning given. In such terrain, pilots need avisual display, including a vertical presentation of the terrain, with aterrain warning to supplement the pull-up alert.

It would therefore be an advantage over the state of the art to providea multi-view display of the position of the aircraft with respect tosurrounding terrain while simultaneously superimposing a radar image ofweather on the same display.

SUMMARY OF THE INVENTION

This invention combines a radar weather display with a terrain elevationdisplay in an easy to read and comprehensive presentation. Terrainelevation and weather information are displayed simultaneously. Thisallows the pilot to choose the best route to avoid high terrain andadverse weather.

An object of the invention is to provide a system that displays terrainelevation as well as aircraft altitude such that the relationshipbetween the terrain and the aircraft can be seen at a glance.

Another object of the invention is to provide a system thatsimultaneously displays terrain elevation, weather conditions, andaircraft position on the same or on selectably separate display screens.

Another object of the invention is to provide a system that depictsterrain and weather information so as to make clear at a glance thedifference between ground returns and adverse weather.

Another object of the invention is to provide on a single displayterrain and weather information and the aircraft position in verticalfrontal and vertical side views in addition to the conventional planview.

Another object of the invention is to provide a system thatdistinguishes between radar weather echoes and radar terrain echoes byblanking all radar returns below a selected elevation stored in a database. This would eliminate or reduce ground clutter, depending on thealtitude selected.

Another object of the invention is to provide a system thatdistinguishes between radar weather echoes and radar terrain echoes byblanking all radar returns above a selected elevation below the terrainelevation stored in a data base.

Another object of the invention is to make conspicuous the terrainelevation that is at, above, or slightly below the altitude of theaircraft and provide for flashing or otherwise highlighting of thesecritical regions.

Another object of the invention is to display terrain elevation witheasy to comprehend contour line mapping image.

Still another object of the invention is to correct for aircraft rolland pitch with the stabilization off.

One embodiment of the invention is a system with an antenna fortransmitting radar signals from the aircraft and for receiving returningradar signals, and a microprocessor for digitizing the returned radarsignals, calculating the latitude and longitude of the locations fromwhich the reflected radar signals were reflected, and a random accessmemory (RAM) for storing the digitized signals and the related latitude,longitude, and altitude. A RAM stores terrain elevation data referencedto latitude and longitude coordinates of the ground over which theaircraft will travel. The RAM also stores an elevation coding table thatassigns a distinctive color and pattern to each range of elevation andthe microprocessor compares the height in the terrain elevation data ateach coordinate with a stored elevation color coding table.

A display RAM receives the data from the microprocessors andsimultaneously displays the data in a plan view image over a horizontalrange. A distinctive color and pattern represents each terrain elevationheight zone on the plan view image. The computer uses the terrainelevation data to generates contour lines on the display. The weatherdisplay and terrain data display are superimposed over one another.

The system has an altimeter input and a microprocessor that compares thealtitude of the aircraft to the terrain elevation data and selects thelatitude and longitude coordinates of critical terrain elevation data.Critical terrain elevation data includes the terrain at a selectedelevation near or above the altitude of the aircraft. The criticalterrain is highlighted on the display. The system may produce an alarmsignal when critical terrain elevation data is selected.

One aspect of the invention is the optional inclusion of a GlobalPositioning System (GPS) receiver in the navigational equipment of theaircraft. In the event that the aircraft is in a position to receivetiming signal from three GPS satellites in low earth orbit, longitudeand altitude information accurate to within a meter is possible. Thisinformation can then be used to coordinate the position of the aircraftwith respect to the internal map stored in memory to enable very preciseplacement of the aircraft with respect to terrain being displayed.Furthermore, if at least four GPS satellites are within range of theaircraft's GPS receiver, altitude information also accurate to within ameter is possible.

The system includes both a vertical front view and at least one verticalside view showing the weather display and terrain data displaysuperimposed over each another.

The system has input from a navigation means for locating the positionof the aircraft and a means for superimposing an aircraft positionmarker at the latitude and longitude coordinates of the present positionover the weather display and terrain data on the plan view image. Anaircraft position marker is superimposed at the determined altitude ofthe aircraft over the weather display and terrain data on the verticalview images.

This system can distinguish between radar weather echoes and radarterrain echoes by blanking all radar returns below a selected elevationabove the terrain elevation stored in a data base. This allows the pilotto exclude radar returns at or below a surface in space that correspondswith terrain but is a chosen altitude above it. This would beparticularly valuable in mountainous country since many thunderstormsdevelop over mountains, making it difficult to distinguish between badweather and high terrain.

Likewise, this system can distinguish between radar weather echoes andradar terrain echoes by blanking all radar returns above a selectedelevation below the terrain elevation stored in a data base. This allowsthe pilot to exclude radar returns at or above a surface in space thatcorresponds with weather but is a chosen altitude below it. This wouldbe useful for search and surveillance radars. The pilot could togglebetween the ground and weather returns quickly and easily.

In one embodiment, the pilot chooses a region at a fixed distance fromthe aircraft, the region having a transverse length and a verticaldistance. A microprocessor continually determines the latitude andlongitude coordinates of locations within the region as the aircraftmoves through space. The microprocessor compares the latitude andlongitude coordinates of the locations within the region to the latitudeand longitude of the stored terrain elevation data and selects thelatitude and longitude of any location within the region that has thesame latitude and longitude as any stored terrain elevation data. Thesystem generates an output signal alarm if the latitude and longitude ofany terrain elevation data is selected and the selected terrainelevation data is highlighted on the display.

In another embodiment, the microprocessor calculates the latitude andlongitude coordinates of the radar echoes even when antennastabilization is not used. The microprocessor directly calculates theantenna slew corrected for bank of the aircraft and the antenna tiltangle corrected for pitch of the aircraft from the antenna slewcorrected for roll, the range or distance of radar echo, the pitch indegrees above or below horizontal, the antenna elevation relative to thehorizontal plane corrected for roll, and the bank angle.

These and other objects, features, advantages and alternative aspects ofthe present invention will become apparent to those skilled in the artfrom a consideration of the following detailed description taken incombination with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of the airborne radar system and theweather and terrain display.

FIG. 2 shows a front view of a cathode ray tube display made with theterrain elevation contoured in the plan view and a correspondingvertical front view.

FIG. 3 is a front view of the cathode ray tube showing an alternativedisplay with a central plan view and a vertical front view at the topand a vertical side view on each side.

FIG. 4a is a partial view of the cathode ray tube showing a portion of avertical display image obtained from a radar scan.

FIG. 4b is a partial view of the cathode ray tube showing a portion ofthe same vertical display image formed from stored terrain elevationdata.

FIG. 4c is a partial view of the cathode ray tube showing a portion ofthe composite image obtained from the radar scan filtered with thestored terrain elevation data.

FIG. 4d is a partial view of the cathode ray tube showing a portion of areversed composite image obtained from the radar scan filtered with thestored terrain elevation data.

FIG. 5a is a graphic representation of the horizontal range of radarreturns with selected voxels highlighted.

FIG. 5b is a graphic representation of the vertical range of radarreturns with a voxel highlighted.

FIG. 6 is a three dimensional graphic representation of the rollcorrections for a banked aircraft with stabilization off.

FIG. 7 is an illustration of how the system can fail to alert a pilotappropriately.

FIG. 8a is a graphic representation of the alternative horizontal rangeof radar returns with voxels highlighted.

FIG. 8b is a graphic representation of the alternative vertical range ofradar returns with voxels highlighted.

FIG. 9 is an illustration of an aircraft using triangulation orquadrangulation from GPS satellites to determine its longitude, latitudeand altitude.

DETAILED DESCRIPTION

FIG. 1 shows one illustrative embodiment of the present invention. Thesystem includes a conventional radar antenna 4 such as a phased arrayflat plate antenna with fixed frontal gain pattern. The antenna 4 ismounted, to oscillate back and forth and direct a beam horizontallyoutwardly, and to move up and down to direct a beam verticallyoutwardly.

Antenna stepper motors 8 are coupled to the antenna 4 to move andposition the antenna in conventional x and y directions of an orthogonalcoordinate system. Horizontal drive control circuit 12 and a verticaldrive control circuit 16 supplies stepper signals directing the antennamotors 8 to move the antenna 4 in a programmable preselected pattern.The horizontal drive control circuit 12 supplies the stepper signals tomove the antenna 4 in the horizontal direction. The vertical drivecontrol circuit 16 supplies the stepper signals to move the antenna 4 inthe vertical direction.

In combination, the two drive control circuits 12 and 16 completelydetermine the pattern and speed at which the antenna 4 moves. Patternsof movement of the antenna 4 are shown in U. S. Pat. No. 4,940,987,which is incorporated herein by reference. The horizontal drive controlcircuit 12 and vertical drive control circuit 16 respond to signals froma stabilization circuit 70 that corrects the position of the antenna 4to compensate for pitch and roll of the aircraft. However, the systemcan correct for aircraft pitch and roll even with the stabilizationcircuit off.

A standard microwave generator, such as a magnetron 20, suppliestransmit signals to the antenna 4. The antenna 4 both transmits radarsignals and receives reflected radar signals. A transmit and receiveswitch 24 allows the flow of electrical signals from the antenna toreceiver circuitry 28 when the antenna is in the receive phase, butblocks the flow of such electrical signals when the antenna is in thetransmit phase. This protects the receiver circuitry 28 fromtransmission energy that might otherwise flow to the receiver circuitrycausing damage.

The receiver circuitry 28 includes a mixer 32 that converts the highfrequency return signals to lower frequency signals suitable forprocessing. A local oscillator 36 supplies an oscillatory signal of acertain frequency to the mixer 32 that produces an intermediatefrequency (IF) signal from the oscillator signal and the incoming returnsignals. An IF receiver 40 adjusts the frequency of the oscillatorsignal to maintain the desired frequency of the IF signal. The IFreceiver 40 amplifies the IF signal received from the mixer 32 andsupplies the amplified signal to a digitizing filter 44. The digitizingfilter 44 converts the received analog signal into a digital signal,typically two or three bytes in length, and filters the digital signalfor transfer to the next stage of the system.

A digital STC (Sensitivity Time Control) correction processor and logicunit 48 is a conventional circuit package that receives digital returndata from the digitizing filter 44 and adjusting the intensity of thedata to compensate for differing distances from which the radar returndata is received (since near return data is more intense than fromdistant return data). The compensated data, including distanceinformation, is then supplied to a central processor 64 that, in turn,supplies it to a special purpose processor 80 for storage in athree-dimensional random access memory 84.

The central processor 64 is a conventional microprocessor that controlsand coordinates the operations of the other circuits and units of theradar system, including a display RAM 52 , a display counter 68, and thestabilization circuit 70. The display RAM 52 is a two-dimensional randomaccess memory in which the central processor 64 stores the picture imagedata (prepared by the special purpose processor 30 to be displayed on acathode ray tube (CRT) 56. This data is supplied to a sweep circuit 60that, in response to the data, produces scan signals for application tothe CRT 56. Return data is supplied to the display RAM 52 to addressesdetermined by the special purpose processor 30 and identified andconditioned for storage by the display counter 68. The display counter68 also addresses the display RAM 52 to refresh the CRT 56 and generatesthe horizontal and vertical retrace trigger signals. Such displayprocedure is known in the radar and video display system art.

The stabilization circuit 70 includes a microprocessor-based subsystemthat monitors signals from a pitch and roll circuit 72 coupled to anaircraft attitude sensor such as a conventional gyroscope 76, andsupplies control signals to the horizontal and vertical drive controlunits 12 and 16. These units, in turn, generate compensating steppersignals for positioning the antenna 4 to account for the pitch and rollof the aircraft detected by the gyroscope 76. The control units 12 and16, control the initial positioning and later movement and speed of theantenna 4, all under control of the central processor 64. Thestabilization circuit 70, pitch and roll circuit 72, and gyroscope 76are-of conventional design and operation.

However, using a conventional stabilization circuit limits the range ofantenna tilt. For example, the maximum manual vertical tilt typically islimited to plus or minus fifteen degree above the horizontal. Withstabilization off the limit may be plus or minus thirty degrees. Thissystem can still correct for roll and pitch, even with the stabilizationoff, by directly supplying to a special purpose processor 80 the rolland pitch angles 72. The special purpose processor uses this data, alongwith the data described below, to calculate the actual position of anobject shown by the radar returns. See FIG. 5 and the related discussionbelow.

The special purpose processor =S receives and stores in thethree-dimensional memory 84 all digital radar return data, includingdistance information, obtained from the antenna 4 sweeping horizontallyin the different sweep planes and tilt angles. The central processor 64supplies the return data received and digitized from each sweep plane tothe special purpose processor on. The special purpose processor updatesthe old data from the sweep plane in the memory 84. The memory 84includes a three-dimensional array of addresses, each for storing datarepresenting a different voxel (volume element) in space from which thecorresponding radar return data was received. The location of the voxelin space and the address in the memory 84 for storing data representingthe voxel is determined from distance information and antenna 4position.

From the stored radar data, the special purpose processor 80 constructsthe two-dimensional display data stored in the two-dimensional displayRAM 52 and displayed on the cathode ray tube 56. This two-dimensionaldisplay includes a horizontal, plan view image with a vertical frontview image and one or more vertical side view images of the terrain andweather condition. The vertical front view is above the horizontal planview, on the display, as shown in FIG. 2 and FIG. 3. The manner ofretrieving and formatting the data, and of selecting the thickness ofthe "slices" of radar returns to be displayed using the cursor controldual knob 136 is fully discussed in U.S. Pat. No. 4,940,987, alreadyincorporated by reference.

FIG. 2 is a front view representation of the cathode ray tube 56 showinga display area screen 112 on which the terrain images are displayed. Ahorizontal plan view image is at 108. Contour lines 109 show terrainelevation in the same way that contour lines show elevation on atopographic map. Critical terrain 110 is highlighted by conspicuouslydisplaying in a contrasting color or pattern within the correspondingcontour lines. Critical terrain is terrain that is at, above, or withina preselected elevation below the altitude of the aircraft. Altitudeshown by an altimeter does not correspond exactly to terrain elevationbecause of barometric pressure, temperature and lapse rate fluctuations,and other variables that affect the altimeter readings. Therefore, asafety margin may be provided by highlighting terrain one or twothousand feet below the assumed altitude of the aircraft. For example,the pilot may choose to highlight terrain with an elevation within 2000feet below the altitude of the aircraft. A vertical front view image isat 104. Critical terrain 129 on the vertical front view image is alsohighlighted.

It should be mentioned at this point that an altitude determining deviceother than an altimeter might be used which is not susceptible to thelimitations inherent in an altimeter. Specifically, a receiver designedto receive timing signals from a Global Positioning System (GPS) ofsatellites might be used to enhance the ability of the invention to havean accurate altitude as well as a longitude and latitude determinationfor the aircraft. This concept of using a GPS receiver will be furtherexplained in connection with FIG. 9 which illustrates use of the GPSsatellite system.

To highlight the critical terrain, the moving map processor 89 receivesthe altitude from the altimeter 86 and signals to the display RAM 95emphasize all pixels where the elevation is equal to the altitude of theaircraft minus the selected margin. Emphasis of a defined area of thedisplay is a common practice in computer graphics. Alternatively, thepilot can enter barometric pressure and the moving map generator can beprogrammed to adjust the altitude measured by the altimeter for thecurrent barometric pressure. Algorithms for such conversion are in wideuse.

With these displays, a pilot flying near critical terrain can determinethe range or distance of the critical terrain from the plan view image108 and the distance or range indicia produced by the cathode ray tube36 from stored distance information. The range indicia are displayedwith control 114. The pilot can determine the elevation of the terrainwith the vertical front view image 104 and altitude indicia 124 and 126produced on the screen from stored altitude information.

The pilot selects the plan view altitude range desired with knobs 116and 118 on the display panel of FIG. 2. Knob 116 sets the upper limit ofthe desired altitude range and knob 118 sets the lower limit. Theoperation of the plan view altitude range is described in detail in U.S.Pat. No. 5,202,690, which is incorporated herein by reference.

An aircraft symbol 128 is on the screen 112 to provide the pilot withthe altitude of the aircraft relative to the indicia 124 and 126. Todevelop the aircraft symbol 128 on the display, an altimeter 86, shownin FIG. 1, measures the altitude of the aircraft and sends thosemeasurements to the special purpose processor 80. The special purposeprocessor stores the readings and produces for display data an image ofthe aircraft 128. The aircraft symbol enables the pilot to quicklyvisualize his elevation relative to terrain and weather into which he isflying as well as the height of the terrain and weather condition abovesea level.

FIG. 3 shows an alternative display with the plan view 212 in thecenter. The vertical, frontal view 204 is directly above it and verticalside view projections 221 and 222 are on each side. The side view on theleft 222 is a vertical view of a trace across the left half of theregion displayed in the plan view. The side view on the right 221 is aview of a trace across the right half of the region. Alternatively, asingle vertical side view could cover the entire region. The side viewdisplays are vertically oriented so the features are displayed at thesame levels as those features in the plan view. These are standardfeatures of orthographic displays.

The scale of the vertical displays in FIG. 3 is changed from FIG. 2 toprovide greater vertical resolution, useful when the aircraft is at alow attitude. As shown in FIG. 3, the aircraft is at 7000 feet but theterrain elevation extends above 10,000 feet. The pilot selects thevertical display scale factor using the inner knob of the TerrainDisplay Control 224.

Although FIG. 2 shows only contoured terrain information, radar and mapimages can be superimposed over the terrain display. Orthographic,multi-planar projections portray the aircraft's height relative toterrain graphically and conveniently. They are standard projections inengineering and architectural drawings. They require no recalculation ofdimensions but only translations and rotations as the aircraft advancesand changes heading.

Another feature of the invention that enables the pilot to find highterrain and adverse weather conditions near the aircraft uses moving mapdisplay technology that has become popular in recent years. See U.S.Pat. No. 5,202,690, already incorporated by reference. The feature ofthe present invention adapts the moving map concept to display not onlymap data but to display both terrain and weather conditions relative tolatitude and longitude so the pilot may view the location of theaircraft relative to terrain and weather.

As shown in FIG. 1, a compass system 88 for determining the heading ofthe aircraft, that is the direction of the long or roll axis of theaircraft, and the altimeter 86 feed both into the special purposecomputer 80 and into a moving map generator 89 continuously. (In FIG. 1,the altimeter 86, the compass system 88, and the navigation system 90are shown in two places.) A navigation system 90, such as LOP-AN, INS(inertial navigation system), VLNF (very low frequency navigationsystem), or GPS (global positioning satellite system), determines theposition of the aircraft and the track of the aircraft over the earth.The navigation system 90 continuously supplies information both to thespecial purpose processor 80 and to the moving map generator 89. Boththe compass system 88 and the navigation system 90 are well known in theaircraft industry.

If the GPS is used, the position of the aircraft as determined therebycan be input to the special processor 8O and the moving map generator 89by multiplexing. The input from a GPS receiver would be accepted if itcan be verified as accurate. GPS receivers have a mechanism forverifying the accuracy known as Receiver Autonomous Integrity Monitoring(RAIM). An additional safety check is also built into the system asdescribed previously by selecting as the altitude of the aircraft thelower of the readings from the altimeter and the GPS receiver. Dependingupon the empirical reliability of the GPS receiver, the GPS couldeventually be selected as primary input which might override or have aweighted preference over an altimeter reading.

The central processor 64 stores in the buffer 91 the plan and verticalview data for a range typically out to a maximum range limit, withcorresponding latitude and longitude addresses for the data. With allthe radar information stored in memory out to the maximum range, a pilotcan effectively view plan or vertical images at any selected rangemerely by actuating the appropriate range selecting switch. Thereforethe buffer provides for full storage of a complete cycle, and rapidlyselective display of a range and thickness.

The plan view data will include a selected altitude "thickness" of radarreturn signals and the vertical view data will include a selectedhorizontal "thickness" of radar return signals. Thus, the plan viewimage will comprise all echoes having latitude and longitude addresseswithin the selected altitude range, and the vertical view image composesall the echoes having the latitude and longitude addresses within theselected horizontal range. The data in the buffer 91, representing datashowing the terrain and weather, combines with moving map data developedby a moving map generator 89.

The moving map generator 89 receives heading information, aircraftposition information, and track information from the compass system 88and navigation system 90, as does the special purpose processor 80. Themoving map generator 89 also accesses and selects information from aterrain elevation data base 94 and from a navigational data base 93 thatcontains latitude and longitude addresses of navigation stations,airports, and way points. The navigational data base 93 and the terraindata base 94 are shown separately although normally they would be onedata base. The terrain data base 94 supplies elevation data for terrainon the earth's surface based on a latitude-longitude grid.

A three dimensional RAM 96 provides memory, consisting of an array ofaddresses, each representing a voxel in space. The highest data pointwithin each voxel is taken to be the voxel elevation. The moving mapgenerator compares each voxel elevation with a look-up table in the RAM96 associated with the moving map generator to determine the color forthe display of that voxel. The look-up table has a different hue foreach range of elevations. These hues are selected to be as similar tothe usual colors of topographic maps as possible without using any ofthe colors regularly used in weather radar displays. The moving mapgenerator also uses the look-up table to create contour lines.Generating lines connecting sequential points is standard techniquewell-known in the computer graphics field. Further, the filling-in ofareas defined by such lines is also well known and common practice.

The look-up table in the moving map generator is also used to comparethe altitude of the airplane with the elevation of the terrain. Theregions having elevations above and within a selected level below thealtitude of the aircraft are highlighted by attention getting signals ashatch marks or alternating flashing hues. The moving map generator alsoprovides an output for an aural alarm, shown as AD in FIG. 1. Thedisplay of hues as a function of altitude and terrain is used in thevertical views also.

The elevation of the highest point in each voxel is also used to definethe terrain surface in the vertical display. The moving map generatorselects the highest elevation in each alignment of pixels, eitherforward or lateral for the region covered in the vertical displays.These elevation points are then connected by lines to form the maximumupper surface of terrain in the region displayed. As with the cursorfunctions, the plan view cursor can be used to define the regiondisplayed during analysis of the terrain profiles.

As shown in FIG. 1, the moving map generator 89 retrieves such latitudeand longitude addresses from the navigational and terrain data bases 93and 94 within a selected distance from the aircraft. The pilot selectsthe distance with control knob 170 shown in FIG. 2. The settinginformation goes to both the special purpose processor 80 and the movingmap generator 89, so both will develop display data over the samelatitude-longitude range. From all of this information, the moving mapgenerator 89 constructs or generates a map display of the navigationstations, airports, and way points, and a contoured terrain mappositioned relative to latitude and longitude, of the area over whichthe aircraft is flying. The moving map generator converts the data to x,y coordinates for display through a second display RAM 95. Alternately,the moving map generator 89 may retrieve data from the buffer 91,converts it to x,y coordinates for display and supplies it directly tothe display RAM 52 rather than through the display RAM 95.

The data from the display RAM 52, containing echo return information,and the display RAM 95, containing terrain elevation and map data,produce on the cathode ray tube 56 a plan view image showing weatherconditions superimposed over terrain contours and map data. The centralprocessor 64 supplies a synchronization signal to the moving mapgenerator 89 as new data is supplied to the buffer 91. The moving mapgenerator 89 can therefore synchronize and coordinate the supply of datato the display RAM 52 and display RAM 95 and the display of suchinformation on the cathode ray tube 56.

As shown in FIG. 2, the system has manual controls to enable selectionof the data displayed on the cathode ray tube 56. These controls includethe display mode control 144, sweep limit controls 148 and 150, alatitude and longitude coordinate push button 154, a heading cursorbutton 158, a scan mode control 166, a tilt angle control 174, and again control 178. These controls, in the form of rotatable knobs orbuttons, are on the display unit 56 (FIG. 1) although the informationand control signals developed by the controls are supplied to thespecial purpose processor 80.

The sweep limit controls 148 and 150, when set, limit the sweep of theantenna in the vertical and horizontal directions respectively. Thesecontrols allow the pilot to narrow and focus on the radar return area.This allows for more frequent updating since the sweep cycle time isreduced if the sweep limits are reduced.

The latitude and longitude button 154, when depressed, signals thespecial purpose processor 80 to provide latitude and longitude gridlines for display on the display screen 112. A push of the latitude andlongitude button 154 causes removal of the latitude and longitude gridlines from the display.

The heading cursor control 158 is a standard feature of conventionalradars and serves to cause the radar system to produce a course headingline on the screen to show a heading for the aircraft.

The controls 162, 166, 170, 174 and 178 are all standard features of aconventional aircraft radar system. Control 162 controls the brightnessof displayed terrain and weather conditions and moving map displayinformation independently of the overall display brightness. Control 166allows the pilot to select the scan mode for the cathode ray tube 156,that is, by stand-by, test, terrain and weather or terrain mapping.

Control 170 selects the range displayed in display 108. Long distances,such as two hundred miles, may supply a general, non-detailed view ofmajor storms ahead, or shorter distances, such as thirty or forty miles,may give a detailed view of a storm. Control 170 sets the latitude andlongitude area for display of both terrain and weather data and movingmap data. Control 174 allows for manually positioning the tilt of theantenna 4 when control 144 is in the "normal" scan mode. Control 178 isan amplifier gain control function. Control 180 is a brightness andintensity control function of the images displayed on the screen 112.

In the plan view of FIG. 2, the contour lines show terrain elevation, ason conventional topographic maps. The aircraft symbol 111 shows theposition of the aircraft relative to the terrain. In the vertical,frontal view the same features are directly above the plan view. Theaircraft symbol 128 is displayed at the aircraft altitude on the samescale.

The system can distinguish between radar weather echoes and radarterrain echoes by blanking all radar returns at a selected elevationabove the terrain elevation data in the terrain data base 94. FIG. 4ashows a portion the vertical display radar image 321 obtained from aradar scan. The radar return from weather conditions and the radarreturn from terrain are indistinguishable. FIG. 4b shows a portion ofthe vertical display terrain image 341 formed by the moving mapgenerator from the data in the data base 93. The system can use theterrain data shown in FIG. 4b to filter the radar scan shown in FIG. 4ato produce the composite image shown in FIG. 4c. The resulting image 365on the display in FIG. 4c excludes the terrain image 341 on the displayin FIG. 4b plus a selected margin of elevation 343 above the terrainimage data.

Likewise, the system can use the terrain data shown in FIG. 4a to filterthe radar scan shown in FIG. 4b to produce the composite image shown inFIG. 4d. The resulting image 367 on the display in FIG. 4c only includesthe terrain image 341 on the display in FIG. 4b plus the selected marginof elevation 343 above the terrain image data. This would be useful forsearch and surveillance radars. The pilot could toggle between theground and weather returns quickly and easily.

The pilot uses the terrain control knob 131 to signal the moving mapgenerator to display the terrain elevation information and to adjust theintensity of the color if displayed. The terrain control knob TR, shownin FIG. 1, is a coaxial, three position switch with input to the centralprocessor 64. In the first position, the terrain control knob signalsthe central processor to turn off the terrain display. In the secondposition, the terrain control knob signals the central processor toenable the terrain display by signaling the moving map generator to loadthe display RAM 95. The terrain control inner knob 133, shown in FIG. 2,controls the color intensity.

In position 3, the terrain control knob signals the central processor toenable the terrain display and to exclude from the radar display allradar returns originating from ground level or within a selecteddistance above the ground level. The selected distance is the selectedmargin of elevation 343 shown in FIG. 4b. The central processor receivesfrom the moving map generator the maximum terrain elevations in eachvoxel and adds to each elevation the value of the selected margin. Thepilot selects the margin by pulling out the terrain control inner knob133 and twisting it. The central processor sends to the display RAM 95the value margin for display on the CRT 56. The central processor 64receives, from the memory 84 through the special purpose processor 80,the radar echoes to be displayed with their addresses. The centralprocessor filters out all pixels with altitude addresses equal to orless than stored terrain elevation plus the margin and sends thecomposite image 365 to the display RAM 52.

FIG. 5a is a graphic representation of the horizontal range (or planview) of radar returns and FIG. 5b is a graphic representation of thevertical range of radar returns. A constant band of voxels 521 ahead ofthe aircraft 523 will maintain a constant relationship with the aircraftand will move with the aircraft relative to terrain. The pilot canselect the transverse length 525 of the band, the vertical distance 527below the aircraft altitude, and the distance between the constant bandand the aircraft 529. In setting this safety zone, the pilot should takeinto account the aircraft speed and its turning radius, plus a safetymargin.

In setting the vertical clearance the pilot will consider the phase offlight, such as cruise over mountainous terrain or the approach segment.The moving map generator compares the vertical clearance altitude of thevoxel band with the maximum terrain altitude within each voxel. If thelatter equals or exceeds the former, an alert is set in the form of anoptional audible warning sound and annunciation. Also, the pixelsregistering the conflict will be indicated in both plan and verticalviews by some conspicuous means such as flashing. After receiving thewarning, the pilot can visualize the potential conflict at a glance atthe display and will also see the best escape route.

An important aspect of the invention should be mentioned in regards tothe phase of flight and the settings of the display. As a practicalmatter, it is unlikely that a pilot will have time sufficient for theselecting of all the display settings in every situation. This is asmuch a matter of the time constraints under which a pilot might beoperating, as well as external conditions such as weather. For example,when landing an aircraft, the pilot is likely to be very occupied payingattention to details of the approach other than the display settings.Therefore, it is desirable that a way be provided to recall apre-programmed flight mode defined by display settings which areparticularly suited to the mode of flight.

For example, suppose that an "approach mode" is designed within a memoryof the display which can later be recalled by the pilot. The approachmode display settings are all selected by the pilot before the flight orthe applicable phase of flight so that a simple control knob can selectall the preprogrammed display settings. A possible approach mode switchcan be an alert knob 152, 153 to be described hereafter.

As shown in FIG. 2, the display mode control 144 provides choices of (1)plan view, (2) plan view and a vertical front view, (3) plan view andthe vertical side views, and (4) plan view with both the vertical frontview and the vertical side views. This control determines the displayformat of the special purpose processor 80, and through it and thecentral processor 64, the display 56.

The pilot uses the outer knob 153 and the inner knob 152, FIG. 2, to setthe terrain warning criteria (alert region). When the outer knob isturned clockwise, past a detent, it enables the inner knob to first setthe width of the pixel zone of warning. A typical setting might be tenmiles if the turning radius of the aircraft at an expected speed is fivemiles. The setting is displayed on CRT 56 at a margin of the field. CRTdisplays of parameter settings are commonly in use and well known in theindustry. Next, the pilot can select the distance ahead of the aircraftthat the warning band should be projected in space, although a beginningpoint forward of the aircraft may not be desirable. An appropriatesetting might also be ten miles. The same knob is used for this settingbut it is pulled out and twisted. Finally, the pilot uses the outer knobto select the altitude clearance desired. The far counter clockwiseposition of the alert knob 152 will turn off the aural alarm. Thecentral processor responds to these control settings by signalling themoving map generator to compare continually the elevations of the datapoints in the selected band of voxels with the aircraft altitude minusthe altitude clearance zone set by control 152. The warning output isgenerated when a maximum elevation in a terrain voxel equals or exceedsthe warning altitude.

In addition to providing an output to an aural alarm or warning device,the moving map generator increases the conspicuousness of the pixelssuch as by flashing. It does so by signalling to display RAM 52 theaddresses of the pixels that are to be emphasized.

The pilot will then receive an audible and/or annunciator alertindicating the need for the pilot to glance at the display screen. On itthe pilot will see the flashing indication in both plan and verticalviews of the location where the potential conflict was discovered. Theindication of that terrain that is near or above the aircraft altitudewill be accentuated as by hatch marks. Thus, the most appropriateavoidance maneuver should be apparent to the pilot at a glance.

The pilot selects scale factor of the vertical displays with the DisplayMode inner knob 146. The display mode inner knob sends the scale throughthe special purpose processor 80 and the central processor 64 to themoving map generator 89. All vertical displays show the weatherconditions and the terrain elevation with the same scale. The criticalterrain highlighted on the vertical view is directly above thecorresponding critical terrain on the plan view.

To select a flight mode, the inner knob 153 is pushed in and turned toselect one of at least three positions: off enroute, or approach. Theselected mode is then shown on the display. After having selected theflight mode, the pilot then pushes in the inner knob 153 momentarily andrepeatedly to thereby cycle through the setting choices of width of thealert region, the distance ahead of region, and distance alert region,and distance beneath the aircraft of the alert region. These selectionsare all shown on the display with the selected flight mode highlightedin reverse video.

The distance and width values are still changed by turning the outerknob 152, for example, to select tens of miles and the inner knob 153 isturned to select single mile increments. The value of the distance seenbelow the aircraft is changed by turning the outer knob 152 to selectthousands of feet and the inner knob to select hundreds of feet above200 feet, and tens of feet below 200. These selected distance and widthvalues are displayed as well as stored in memory. The values are used tocommand the microprocessor 89 of the moving map generator to set thealert parameters, store them in the 3 D memory 96, and then send them tothe display RAM 95 for display on the CRT 56.

To switch between an enroute flight mode and an approach flight mode,the pilot pushes in the inner knob 153 and turns it to make the flightmode selection. It should be realized that coaxial switches such asinner knob 153 and outer knob 152 are well known to those skilled in theart.

An aspect of the invention described above which might not beimmediately apparent occurs when the aircraft is descending rapidly. Asstated previously, it is possible to preselect how far the alertdistance begins ahead of the aircraft as shown in FIGS. 5a and 5b. Ifthis alert distance were relatively large because of the flight mode, itmight be possible for the aircraft to encounter terrain without anywarning.

To illustrate this scenario, suppose for example that the aircraft 700in FIG. 7a was descending rapidly as indicated by arrows 702 inmountainous terrain and the pilot has not yet switched to the approachflight mode. Local terrain 704 could appear which is between theaircraft 700 and the beginning of the preselected alert distance asrepresented by the line of voxels 706. Therefore, it appears prudent tomodify all alert regions of the flight modes such that the warningregion always begins and extends from directly below and forward fromthe aircraft as in FIG. 7b.

FIG. 7B shows the aircraft 700 as it now approaches the same terrain704. By shortening the distance between the aircraft 700 and thebeginning of the voxels 706, there is no opportunity for terrain 704 toarise unexpectedly or without warning.

This concept of expanded alert region is illustrated in FIGS. 8a and 8b.FIG. 8a shows that instead of the single row of voxels, the alert regionis now defined as a horizontal plane 800 in space at a selected levelbelow the aircraft 808. Always beginning the warning region directlybeneath the aircraft 808 a selectable distance 802 and then forwardtherefrom a selectable distance 800 will prevent the described scenarioof FIG. 7a from occurring. The pilot could still select the width 804 ofthe alert region, the distance forward 800, as well as the distancebelow the aircraft 808 where the alert region would begin. The onlyfactor removed from pilot control is how far forward of the aircraft 808the alert region could begin. In essence, the alert region would neverstart at some distance forward of the aircraft 808, but instead wouldalways begin some distance generally directly below the aircraft 808.

In practice, an alert will be reported whenever any elevation of themoving map within the preselected alert region equals or exceeds thealtitude of the alert region. However, to further enhance thereliability of the system described above, the aircraft can be describedas an object in memory having a thickness of at least two voxels. Thiswould prevent a corrupted single voxel position stored in memory fromresulting in erroneous alert messages or more importantly, the lackthereof.

In operation, the system might alert the pilot that the flight should bechanged from the enroute mode to the flight mode so that appropriatealert criteria are being used. Even after changing flight modes bybringing up preselected flight mode settings from memory, the settingscan always be changed as necessary.

Another embodiment of the invention which can have great utility is analternate display mode. In this new display mode, the height of theaircraft above the highest point of terrain in the region would bedisplayed numerically. This numerical readout could be actuated by a newswitch added to the display controls shown in FIGS. 2 and 3.

The function of the numerical display serves the same function as aradio altimeter with several important differences. First, the displayedvalues will not be limited to a maximum altitude above terrain of about2500 feet as is the case with present radio altimeters. Furthermore, thenumerical display will show the altitude above the terrain not only ofthe aircraft, but also the projected distance ahead of the aircraft ofthe highest local terrain. If the pilot has set the width and forwarddistance of the warning region to 0, it should function as a radioaltimeter, thereby displaying the height of the aircraft above groundlevel. Another difference is that the numerical display will give truevertical distance reports, whereas radar altimeters give somewhat falsehigh readings if the aircraft is banking. In contrast with the presentinvention, in steep banking maneuvers, the radar altimeter might provideno signal at all.

Another aspect of the invention is the interchangeability of theinformation displayed on various instrument display screens. It shouldbe readily apparent that although the control switches do not move, theactual displayed information might be switched to other displays asdesired.

FIG. 3 shows the screen with terrain features in the vertical, frontview directly above the corresponding terrain features in the plan view.The vertical, side view displays those terrain features directly abeamthe corresponding plan view features. The left side view include thefeatures of the left half of the plan view and the right side view thefeatures of the right half. As shown, the vertical display scale hasbeen changed by the display mode inner knob 146 to give a maximum rangeof 10,000 feet. The aircraft symbol 228 is at an altitude of 7,000 feetand the terrain extends above 10,000 feet.

The system's displays do not show exact distances of the aircraft abovethe terrain as the display scales are gross. This system shows generallythe location of threatening terrain rather than provide clearanceinformation. This system supplements, rather than replaces, more precisewarning devices such as the GPWS.

FIG. 6 shows a three dimensional graphic representation of the rollcorrections for a banked aircraft with stabilization off. As notedabove, this system can correct for roll and pitch with the stabilizationoff by directly supplying to a special purpose processor 80 the roll andpitch angles 72. The special purpose processor uses the pitch and rollangles, along with the antenna position relative to the aircraft andecho range, to calculate the actual position of an object shown by theradar returns.

In FIG. 6, the solid lines represent the aircraft level, the dashedlines represent the aircraft banked by B degrees. Also shown are theradar echo position E, the antenna slew A_(b) in banked plane, the rangeR or distance to radar echo, the antenna elevation relative to thehorizontal plane Θ_(b) displaced by roll, the vertical distance V_(b) toradar echo from horizontal plane displaced by roll angle, the distanceK_(b) from roll axis to V_(b), the vertical distance V_(h) from radarecho to horizontal plane, the distance K_(h) from roll axis to V_(h),the horizontal distance R_(h) from aircraft position to plane of radarecho perpendicular to roll axis, the distance J from echo to roll axis,the angle D between J and vertical line from radar echo and horizontaldisplaced by roll angle, the distance H in horizontal plane formaircraft position to base of perpendicular line through radar echo, thedistance H_(b) in horizontal plane displaced by roll angle from aircraftposition to base line through the radar echo that is perpendicular todisplaced horizontal plane, and antenna tilt angle Θ corrected for pitchof the aircraft.

As described in U.S. Pat. 5,202,690, the special purpose processor 80can calculate the latitude E, longitude E' and altitude V relative toaircraft for each weather echo if supplied the track T, crab angle C,horizontal antenna deflection A, range to the echo R, and the antennatilt angle Θ, as follows: ##EQU1## With the stabilization on, thehorizontal antenna deflection A and the antenna tilt angle Θ arecorrected for aircraft attitude. With the stabilization off, the specialpurpose processor 80 can calculate the horizontal antenna deflection Aand the antenna tilt R from the pitch and roll information using thefollowing formulas: ##EQU2## where A antenna slew corrected for bank ofthe aircraft;

Θ antenna tilt angle corrected for pitch of the aircraft;

A_(b) antenna slew uncorrected for roll (banked);

R range of radar echo;

P pitch in degrees above or below horizontal;

Θ_(b) antenna elevation relative to the horizontal plane uncorrected forroll (banked);

B bank angle.

Another aspect of the present invention is-a potential improvement indetermining the longitude, latitude and altitude of the aircraft a shownin FIG. 9. This is accomplished by taking advantage of the United StatesGovernment's Global Positioning System (GPS) of low earth orbitsatellites. A boon to precise location determining occurred when theUnited States saw fit to invest over $12 Billion in creating a networkof 24 satellites in low earth orbit, each broadcasting precise timingsignals from two onboard atomic clocks. Using precise and well-developedtriangulation and quadrangulation formulas, a receiver mounted in anaircraft 902 that picks up signals from several satellites 900simultaneously can determine its position in global coordinates, namelylatitude, longitude and elevation if the signals can be received fromfour GPS satellites 900 simultaneously.

With this network orbiting overhead, a person anywhere on the earth hasa 24 hour a day line-of-sight view to a sufficient number of satellitessuch that a GPS receiver is able to determine longitude, latitude andelevation to within several meters. However, the Department of Defensehas only recently announced that intentional errors introduced into theGPS timing signal are to be removed. The purpose of intentional errorintroduction, known as Selective Availability (SA), was to prevent aforeign power from being able to more precisely target weapons systemsusing the United States own position determining system. With theremoval of SA, the accuracy of GPS can provide a much more accurateposition determining means.

However, it should be realized that an aircraft does not want to abandoncurrent methods of position determination. One reason is that the GPSsystem might not always be available if, for instance, the aircraft wereto descend into a valley having high walls which would prevent directline of sight to at least three GPS satellites. Therefore, currentmethods of position determination would continue to be used as backupwhen GPS was not functioning.

One strategy for using all the available position data of the aircraftmight be to poll a conventional position determining device and the GPSreceiver in the aircraft to thereby determine the lowest altitude. Thesystem could then be designed to base all calculations for display basedon the lower of the two altitude readings for the aircraft. This processwould add an extra safety margin to the system.

FIG. 9 illustrates the concept of the Global Positioning System (GPS) asdescribed above. A GPS receiver 900 in the aircraft receives timingsignals from at least three, and preferably four low earth orbitingsatellites 902, 904, 906 and 908. The timing signals are provided byextremely accurate atomic clocks in the satellites, two redundant clocksaboard each satellite providing backup. Three satellites providesufficient information for a GPS receiver 900 to calculate a longitudeand latitude using triangulation formulas well known to those skilled inthe art. If a signal can be received from four satellites, the altitudeof the GPS receiver 900 can also be determined using a modifiedtriangulation formula.

The above described arrangements are only illustrative of theapplication of the principles of the present invention. Numerousmodifications and alternative arrangements may be devised by thoseskilled in the art without departing from the spirit and scope of thepresent invention and the appended claims are intended to cover suchmodifications and arrangements.

I claim:
 1. A selectable weather radar and terrain map display systemfor aircraft comprising:emitting and receiving means for emitting radarsignals from the aircraft and for receiving back reflected radarsignals; means for digitizing the reflected radar signals received bythe receiving means; means for calculating latitude and longitudecoordinates of features which reflected the radar signals; means forstoring a plurality of flight modes having storable and retrievablealert region settings which correspond to a region of space relative tothe aircraft which can result in an alarm when features are within thealert region; first means for storing the digitized signals and thelatitude and longitude coordinates of the features; second means forstoring terrain elevation data referenced to latitude and longitudecoordinates of the ground over which the aircraft will be traveling; anddisplay means, responsive to the first and second storing means, forsimultaneously displaying a plan view image indicating superimposedweather and terrain elevation data represented by the digitizedreflected radar signals relative to the calculated latitude andlongitude coordinates.
 2. The system as in claim 1 furthercomprising:means for determining altitude of the aircraft; means forcomparing the altitude of the aircraft to the terrain elevation data tothereby select latitude and longitude coordinates of critical terrainelevation data defined as terrain which is at a selected level asdefined by the selectable alert region with respect to the altitude ofthe aircraft; and means for highlighting an image of the criticalterrain elevation data on the display means.
 3. The system as in claim 2wherein the means for determining the altitude of the aircraft comprisesa Global Positioning Satellite (GPS) receiver which receives timingsignals from at least four GPS satellites.
 4. The system as in claim 3wherein a latitude and longitude of the aircraft is determined using theGPS receiver which receives timing signals from at least three GPSsatellites.
 5. The system as defined in claim 1 wherein the systemfurther comprises a plurality of flight modes having storable andretrievable selected radar settings and corresponding altitude levelswhich are rapidly actuable depending upon a flight mode of the aircraft,such that an operator is not distracted by having to enter specificradar settings during any flight mode.
 6. The system as defined in claim5 wherein the plurality of flight modes include an enroute flight modeand an approach flight mode.
 7. The system as defined in claim 6 whereinthe plurality of flight modes are defined by a width of the alertregion, a length of the alert region, a distance beneath the aircraftfrom where the alert region begins, and a thickness of the alert regionin a vertical direction, wherein the length is defined as a distanceextending forwardly from directly beneath the aircraft.
 8. The system asdefined in claim 7 wherein the display means includes at least onenumerical display of a height of the aircraft above a highest point ofterrain in the alert region.
 9. The system as defined in claim 8 whereinthe at least one numerical display shows the height of the aircraftabove a plurality of highest points of terrain in the alert region. 10.The system as in claim 2 further comprising means for producing an alarmsignal when the critical terrain elevation data is selected.
 11. Thesystem as in claim 1 wherein the display means simultaneously displays avertical and/or horizontal view indicating superimposed weather andterrain elevation data represented by the digitized reflected radarsignals relative to the calculated latitude and longitude coordinates.12. The system as in claim 11 wherein the display means forsimultaneously displaying a vertical view includes:a vertical front viewof the weather and the terrain elevation data represented by thedigitized reflected radar signals relative to the calculated latitudeand longitude coordinates; and at least one vertical side view of theweather and the terrain elevation data represented by the digitizedreflected radar signals relative to the calculated latitude andlongitude coordinates.
 13. The system as in claim 1 furthercomprising:third means for storing an elevation coding table assigning adistinctive color and pattern to each range of elevation, and means forcomparing the height in the terrain elevation data at each of thelatitude and longitude coordinates with the stored elevation colorcoding table, and assigning a distinctive color and pattern to representa terrain elevation height for the latitude and longitude coordinates onthe plan view image.
 14. The system as in claim 1 further comprisingmeans for retrieving the terrain elevation data relative at eachlatitude and longitude coordinate to thereby generate contour lines onthe display means.
 15. The system as in claim 1 further comprisingnavigation means for locating a present position of the aircraft and forsuperimposing an aircraft position marker at the latitude and longitudecoordinates of the aircraft's present position over the weather andterrain elevation data on the plan view image.
 16. The system as inclaim 15 further comprising:means for determining the altitude of theaircraft, means for superimposing an aircraft position marker at thedetermined altitude of the aircraft over the weather display and terraindata on the vertical view image of the display means.
 17. The system asin claim 1 further comprising:third means for storing map datareferenced to the latitude and longitude coordinates of the ground overwhich the aircraft will be traveling; display means, responsive to thefirst, second, and third storing means, for simultaneously displaying aplan view image indicating superimposed weather and terrain elevationdata represented by the digitized reflected radar signals relative tothe calculated latitude and longitude coordinates, and map data relativeto latitude and longitude coordinates of the map data.
 18. The system asin claim 1 further comprising:a region at a fixed distance from theaircraft, the region having a transverse length and a vertical distance,means for continually determining the latitude and longitude coordinatesof locations within the region as the aircraft moves through space, andmeans for comparing the latitude and longitude coordinates of thelocations within the region to the latitude and longitude of the storedterrain elevation data and for selecting the latitude and longitude ofany location within the region that has the same latitude and longitudeas any stored terrain elevation data.
 19. The system as in claim 18wherein the region begins at a point directly beneath the aircraft andextending outwardly and forwardly of the aircraft.
 20. The system as inclaim 18 further comprising means for generating an output signal whenthe latitude and longitude of the terrain elevation data is selected.21. The system as in claim 18 further comprising means for highlightingon the display the latitude and longitude of all the selected terrainelevation data.
 22. The system as in claim 1 wherein the means forcalculating the latitude and longitude coordinates includes means forcalculating the latitude and longitude coordinates when antennastabilization is not used.
 23. The system as in claim 1 further whereinthe means for calculating the latitude and longitude coordinates furthercomprises a means for calculating the antenna slew corrected for bank ofthe aircraft A and the antenna tilt angle corrected for pitch of theaircraft Θ, given the antenna slew uncorrected for roll A_(b), the rangeor distance of radar echo R, the pitch in degrees above or belowhorizontal P, the antenna elevation relative to the horizontal planeuncorrected for roll Θ_(b) and the bank angle B as follows: ##EQU3## 24.A weather radar and terrain map display system for aircraftcomprising:emitting and receiving means for emitting radar signals fromthe aircraft and for receiving back reflected radar signals; means fordigitizing the reflected radar signals received by the receiving means;means for calculating latitude and longitude coordinates of featureswhich reflected the radar signals; first means for storing the digitizedsignals and the latitude and longitude coordinates of the features;second means for storing terrain elevation data referenced to latitudeand longitude coordinates of the ground over which the aircraft will betraveling; means for storing a plurality of flight modes having storableand retrievable alert region settings which correspond to a region ofspace relative to the aircraft which can result in an alarm whenfeatures are within the alert region; filtering means, responsive to thefirst and second storing means, for comparing the digitized radarsignals with the terrain elevation data at the same latitude andlongitude as the latitude and longitude calculated for the digitizedradar signal and blocking those digitized radar signals where calculatedelevation of the radar signal is below the corresponding terrainelevation. display means, responsive to the filtering means, fordisplaying a plan view image over a range of weather represented by thefiltered digitized reflected radar signals relative to the calculatedlatitude and longitude and elevation coordinates.
 25. The system as inclaim 24 wherein the display means, responsive to the filtering means,displays a vertical view image of weather, represented by the filtereddigitized reflected radar signals, relative to the calculated latitudeand longitude and elevation coordinates.
 26. The system as in claim 24further comprisinga region at a fixed distance from the aircraft, theregion having a transverse length and a vertical distance, means forcontinually determining the latitude and longitude coordinates oflocations within the region as the aircraft moves through space, andmeans for comparing the latitude and longitude coordinates of thelocations within the region to the latitude and longitude of the storedterrain elevation data and for selecting the latitude and longitude ofany location within the region that has the same latitude and longitudeas any stored terrain elevation data.
 27. The system as in claim 26further comprising means for generating an output signal when thelatitude and longitude of the terrain elevation data is selected. 28.The system as in claim 26 further comprising means for highlighting onthe display the latitude and longitude of all the selected terrainelevation data.
 29. The system as in claim 24 wherein the means forcalculating the latitude and longitude coordinates includes means forcalculating the latitude and longitude coordinates when antennastabilization is not used.
 30. The system as in claim 24 further whereinthe means for calculating the latitude and longitude coordinates furthercomprises a means for calculating the antenna slew corrected for bank ofthe aircraft A and the antenna tilt angle corrected for pitch of theaircraft Θ, given the antenna slew uncorrected for roll A_(b), the rangeor distance of radar echo R, the pitch in degrees above or belowhorizontal P, the antenna elevation relative to the horizontal planeuncorrected for roll Θ_(b) and the bank angle B as follows: ##EQU4## 31.A terrain map display system for aircraft comprisingmeans for storingterrain elevation data referenced to latitude and longitude coordinatesof the ground over which the aircraft will be traveling, and displaymeans, responsive to the storing means, for displaying a plan view imageover a horizontal range showing the terrain elevation data relative tothe latitude and longitude coordinates of that data.
 32. The system asin claim 31 further comprisingmeans for determining the altitude of theaircraft, means for comparing the altitude of the aircraft to theterrain elevation data and selecting the latitude and longitudecoordinates of the critical terrain elevation data where the elevationof the terrain is at a selected level with respect to the altitude ofthe aircraft, and means for highlighting the image of the selectedcritical terrain elevation on the display means.
 33. The system as inclaim 31 further comprising means for producing an alarm signal whencritical terrain elevation data is selected.
 34. The system as in claim31 wherein the display means, responsive to the storing means, displaysa vertical view showing the terrain elevation data relative to thelatitude and longitude coordinates of that data.
 35. The system as inclaim 33 wherein the display means, responsive to storing means, fordisplaying a vertical view includes:a vertical front view of terrainelevation data relative to the latitude and longitude coordinates ofthat data; and at least one vertical side view of terrain elevation datarelative to the latitude and longitude coordinates of that data.
 36. Thesystem as in claim 31 further comprising:fourth means for storing anelevation coding table assigning a distinctive color and pattern to eachrange of elevation; and means for comparing the height in the terrainelevation data at each coordinate with the stored elevation color codingtable, and assigning distinctive color and pattern to represent theterrain elevation height at each coordinate on the plan view image. 37.The system as in claim 31 further comprising means for retrieving theterrain elevation data relative at each coordinate to generate contourlines on the display means.
 38. The system as in claim 31 furthercomprising navigation means for locating the present position of theaircraft and of superimposing an aircraft position marker at thelatitude and longitude coordinates of the present position over theterrain data on the plan view image of the display means.
 39. The systemas in claim 31 further comprising:means for determining the altitude ofthe aircraft; and means for superimposing an aircraft position marker atthe determined altitude of the aircraft over the terrain data on thevertical view image of the display means.
 40. The system as in claim 31further comprising:means for receiving radar signals with the latitudeand longitude coordinates calculated for the radar signals; and whereinthe display means, also responsive to the means for receiving radarsignals, simultaneously displays (a) weather, represented by digitizedradar signals, relative to the calculated latitude and longitudecoordinates, and (b) terrain elevation data relative to the terrainelevation data relative to the latitude and longitude coordinates ofthat data, said weather display and terrain data display beingsuperimposed over one another.